1. Field of the Invention
This invention relates in general to methods and apparatus for measuring earth formation dip and in particular to methods and apparatus for measuring earth formation dip which utilize sonic techniques.
2. Description of the Prior Art
The measurement of dip in earth formations is generally performed by comparing logs obtained by at least three separate pads mounted on a sonde and facing angularly spaced portions of a borehole wall. The measurements obtained are resistive in nature and are largely immune to the effects of borehole rugosity and mudcake buildup. Due to the resistive nature of the measurement involved it is necessary to have an excellent conductive interface between the pad and the formation. This condition is not possible in boreholes in which oil based mud is utilized. Field experience has shown that oil based mud forms a thin irregular layer of insulation between the pad and the borehole wall which prevents electrical current from entering the formation. The only known attempt in the prior art to solve this problem utilizing resistive measurement techniques involves the implementation of spines or "scratchers" on each pad to establish electrical conductivity with the formation. This has not proven to be a successful solution.
U.S. Pat. No. 3,388,323 attempts to solve the problem of oil based mud measurements by measuring dip utilizing magnetic measurements. Still other attempts at solving this problem have utilized acoustic waves which are propagated through the geological formations along a direction substantially parallel to the borehole. Examples of this type of dip measurement are disclosed in U.S. Pat. Nos. 4,495,605; 3,526,874; 3,376,950 and 3,295,100. None of the aforementioned systems have met with commercial success. Therefore, it should be obvious that a need exists for a dip meter capable of operation in highly resistive oil based muds.